Thermistors are devices having a temperature varying electrical resistance. If the resistance decreases with increasing temperature, the devices are generally referred to as negative temperature coefficient (NTC) thermistors. These devices, i.e., NTC thermistors, are widely used for temperature measurement as well as for temperature control and compensation of other circuit elements which have positive temperature coefficients of resistivity. Many types of materials, for example, oxides and semiconductors, are used in thermistors and are useful in resistance measuring devices at temperatures as low as, for example, liquid helium temperatures, and as high as, for example, 1600 degrees K.
While many characteristics of thermistors are of interest, one principal device characteristic of interest is the room temperature resistance. This resistance may range from several ohms to several megohms. Both the device geometry and thermistor material characteristics affect the resistance. The resistivity versus temperature behavior for negative temperature thermistors generally follows an exponential relationship over a wide range of temperatures, that is, .rho.=A exp (.beta./T) where .rho. is the resistivity at temperature T, A is a constant over the temperature range of interest and .beta. is the temperature coefficient. Knowledge of the room temperature resistivity and thus, the resistance, and the value of .beta. permits calculation of the device resistance at any temperature within a specified temperature range. A high value of .beta., for a given resistance, is generally desirable as it provides a resistance that changes rapidly with temperature.
Other device characteristics may be important for some applications. For example, if joule heating of the thermistor is utilized, specification of the heat dissipation rate will be necessary. The heat dissipation rate, generally specified as the number of milliwatts required to raise the thermistor temperature by 1 degree C., depends upon the device geometry and may be varied by changing the device geometry. Additionally, a thermal time constant which specifies the response time of a thermistor to a change in temperature may be important for some applications. The thermal time constant depends on the thermal mass of the device.
Many, if not most, negative temperature coefficient thermistors are fabricated from the oxides of transition metals such as manganese, nickel and cobalt. See, for example, Semiconducting Temperature Sensors and Their Applications, pp. 59-87, Herbert B. Sachse, John Wiley and Sons, New York, 1975. The increasing cost of some transition metals, especially cobalt, makes alternative and lower cost materials desirable. Cobalt lacking ferrites, such as ZnFe.sub.2 O.sub.4, have been investigated for use in negative temperature coefficient thermistors but such ferrites tend to have .beta. values that are significantly, and undesirably, lower than the .beta. values of thermistors based on the NiMnCo system for the desired device resistances. As a result, the latter system has been more widely used commercially. Even for the cobalt based system, however, higher .beta. values would be desirable for some purposes.
Negative temperature coefficient thermistors are generally formed by mixing, calcining and ball milling oxides or carbonates of the transition metals to form a powder suitable for fabrication into thermistors. Fabrication may be done by dry pressing as well as other techniques that use a slurry of the powder. For all fabrication methods, however, the materials are sintered to their final form.